In general, in order to manufacture a desired semiconductor integrated circuit, various thermal processes including a film-forming process, an etching process, an oxidation process, a diffusion process, a modifying process or the like are carried out to a semiconductor wafer, which consists of a silicon substrate or the like. For example, as an oxidation process, there are known an oxidation process that oxidizes a surface of a single-crystal silicon film or a poly-silicon film, and another oxidation process that oxidizes a metal film, and so on. Such an oxidation process is mainly used for forming an insulation film such as a gate oxide film or a capacitor.
In view of a pressure, there are a normal-pressure oxidizing method that is carried out in a processing container under an atmosphere substantially the same as the atmospheric pressure, and a reduced-pressure oxidizing method that is carried out in a processing container under a vacuum. In addition, in view of a kind of gas used for the oxidation process, there are a wet oxidizing method that uses moisture vapor generated by burning hydrogen and oxygen by means of an outside burning unit (for example, JP A 3-140453), and a dry oxidizing method that causes only ozone or oxygen to flow into a processing container without using moisture vapor (for example, JP A 57-1232).
Herein, taking into consideration film characteristics as an insulation film such as pressure resistance, corrosion resistance, reliability or the like, in general, an insulation film formed by a wet oxidizing process is superior to an insulation film formed by a dry oxidizing process.
In addition, in general, an oxide film formed by a wet oxidizing process under a normal pressure can achieve a great oxidation rate, but is inferior in uniformity within a surface of a film thickness. On the other hand, an oxide film formed by a wet oxidizing process under a reduced pressure can achieve only a small oxidation rate, but is superior in uniformity within a surface of a film thickness.
Conventionally, design rules for a semiconductor integrated circuit were not so severe. Thus, the above various oxidizing methods were suitably selected taking into consideration use application of the oxide film, process condition for forming the oxide film, apparatus cost for forming the oxide film or the like.
However, recently, a wire width and/or a film thickness have been decreased so that the design rules for a semiconductor integrated circuit have become more severe. Thus, better film characteristics and/or higher uniformity within a surface of a film thickness have been required. The conventional oxidizing methods can not cope with the requests sufficiently.
An example of a wet oxidizing method is disclosed in JP A 4-18727. In this example, an H2 gas and an O2 gas are separately introduced into a lower-end portion of a longitudinal quartz reaction tube, and then burned at a burning part provided in a quartz cap. Moisture vapor is generated by the burning reaction. The moisture vapor rises up along an arrangement direction of wafers and oxidizes the wafers. In the case, the H2 gas is burned at the burning part, so that the lower-end portion of the processing container becomes rich in the moisture vapor. As the moisture vapor rises up, the moisture vapor is consumed. Thus, to the contrary, an upper-end portion of the processing container becomes short in the moisture vapor. Thus, film thicknesses of the oxide films formed on the wafers may be greatly different depending on supporting positions of the wafers. That is, uniformity between surfaces of a film thickness of the oxide films may be deteriorated.
In addition, in an oxidizing unit disclosed in JP A 57-1232, a plurality of semiconductor wafers are arranged alongside in a horizontal batch type of reaction tube. An O2 gas may be solely introduced into an end portion of the reaction tube. Alternatively, an O2 gas and an H2 gas may be simultaneously introduced thereinto. Then, an oxide film is generated under a reduced-pressure atmosphere. However, in the conventional unit, the film-forming process is carried out by using a hydrogen-burning oxidizing method under an atmosphere whose pressure is relatively high. That is, the moisture vapor is main in the reaction. Thus, as described above, density difference of the moisture vapor may be generated between an upstream side and a downstream side of the gases in the processing container. Thus, uniformity between surfaces of a film thickness of the oxide films may be deteriorated.
In addition, in a unit disclosed in the specification of U.S. Pat. No. 6,037,273, an oxygen gas and a hydrogen gas are supplied into a single-wafer type of process chamber, which is heated by a lamp. The both gases react in the vicinity of a surface of a semiconductor wafer arranged in the process chamber so as to generate moisture vapor. The moisture vapor oxidizes silicon of the wafer surface, so that an oxide film is formed.
However, in the unit as well, the oxygen gas and the hydrogen gas are introduced into the process chamber from gas ports which are located away from the wafer by about 20 to 30 mm, and the process pressure is relatively high. Thus, uniformity within a surface of a film thickness is inferior.
In order to solve the above problems, JP A 2002-176052 by the applicant discloses an oxidizing method wherein an oxidative gas such as an O2 gas and an reducing gas such as an H2 gas are simultaneously supplied into an upper portion and a lower portion of a process chamber respectively, and react on each other under a vacuum atmosphere in order to form an atmosphere mainly consisting of oxidation active species and active hydroxyl species. In that atmosphere, the silicon wafer or the like may be oxidized.
The oxidizing method is explained simply with reference to FIG. 9. FIG. 9 is a schematic structural view showing an example of a conventional oxidizing unit. The oxidizing unit 102 shown in FIG. 9 has a longitudinal cylindrical processing container 106. A resistance heater 104 is arranged around the processing container 106. In the processing container 106, a wafer boat 108 is arranged, which can be moved up and down in order to be loaded and unloaded through a lower end of the processing container 106. Semiconductor wafers W consisting of silicon substrates or the like are placed and held on the wafer boat 108 in a tier-like manner. An H2-gas nozzle 110 for supplying an H2 gas and an O2-gas nozzle 112 for supplying an O2 gas are provided at a lower side wall of the processing container 106. A gas-discharging port 114 connected to a vacuum pump not shown or the like is provided at an upper portion of the processing container 106.
The H2 gas and the O2 gas introduced into (a lower portion of) the processing container 106 from the both nozzles 110, 112 react on each other in the processing container 106, for example under a pressure smaller than 133 Pa, in order to generate active oxygen species and active hydroxyl species. These active species rise up in the processing container 106, come in contact with surfaces of the wafers W, and oxidize the surfaces.
According to the oxidizing methods disclosed in the above six documents, an oxide film having good film characteristics can be formed, and the uniformity within a surface of a film thickness of the oxide film can be maintained high.
However, according to the oxidizing methods disclosed in the above six documents, density of the active species is high on an upstream side of the gas flow, but low on a downstream side thereof, so that uniformity between surfaces of a film thickness is very low, which corresponds to a degree of film-thickness difference between the wafers.
In addition, recently, use application of the semiconductor integrated circuit has been widened, so that tendency of many-kind small-volume production has become stronger. That is, when a maximum containing capacity for production wafers of the wafer boat 8 is about 50 to 150, some process may be carried out under a state wherein production wafers whose number is smaller than the capacity are contained.
When the production wafers are short, it is not preferable to carry out an oxidation process under a process condition for a normal full-wafer state (gas flow rate or the like) while one or more vacant areas remain. The production wafer has a very large surface area because a pattern is formed on a surface thereof. Thus, the production wafer tends to consume a large amount of the active species. The degree of consumption of the active species is called a loading effect. Because of the loading effect, depending on a containing manner of the production wafers in the wafer boat (containing number and/or containing positions), distribution and/or an amount of the active species may be greatly changed. This may have a bad effect on the uniformity between surfaces of a film thickness.
Then, in general, when the production wafers are short, dummy wafers are used to fill the wafer boat 108. Thus, conditions in the processing container 106 such as temperature distribution and/or gas flows are maintained at substantially the same as the normal full-wafer state.
However, the dummy wafers are relatively expensive. Thus, production cost may be increased thereby.
The inventor has studied to compensate a change of heat distribution caused by the vacant areas by a temperature adjustment of heating units that can be controlled independently for each zone, when the vacant areas remain in the wafer boat 108 without using the dummy wafers. However, in the case, thermal history of the production wafers may be changed. Thus, it is difficult to adopt this manner.